Printing system and printing method

ABSTRACT

The colors used for forming an image are limited in advance to a predetermined number of color patterns called representative colors; on the host side the image data received is divided into blocks, for each of which a representative color is determined in a representative color code conversion unit; the representative colors and their associated representative color codes are transmitted to the printer side; and on the printer side the representative color codes are converted into dot patterns in a pattern conversion unit for each pass before being output. The above procedure allows a series of processing—resolution conversion, masking/UCR processing, output γ-correction and binarization processing—to be realized with a simple configuration.

This application is based on Patent Application No. 2000-323197 filedOct. 23, 2000 in Japan, and is a divisional application of U.S. patentapplication Ser. No. 09/983,102, filed Oct. 23, 2001, and claims benefitof the filing dates of those applications under 35 U.S.C. §§ 119 and120, respectively. The content of both mentioned prior applications isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a printing apparatus for and a printingmethod of forming pixels with dot patterns and more specifically toimage processing in a printing apparatus that performs a so-calledmultipass printing in which the same line is printed in a plurality ofscanning operations of a print head each using different nozzles.

2. Description of the Prior Art

As information processing equipment such as computers have come intowide use in recent years, printing apparatus, such as printers, that areperipheral devices of the computers are also becoming rapidlywidespread. At the same time as the general trend for higher quality andcolor representation of visual information in the information processingequipment accelerates, there are also growing demands on the printingapparatus for a higher print quality and for a color printingcapability.

A well-known printer image processing method is an image processingtechnique based on a color halftone model.

FIG. 13 is a block diagram showing an example of a conventionalimage-processing unit used in a printer.

Image data in the form of an RGB signal is input from an input terminal101, converted into a resolution level compatible with that of theprinter by a resolution conversion unit 102, and then converted into aCMY density signal for each pixel by a density conversion unit 103. Theimage data, after being converted from the RGB signal into the CMYdensity signal, is further converted by a black generation unit 104 intoa density signal that includes a black density signal K. The image datanow in the form of a CMYK density signal is subjected to under colorremoval processing and masking in a masking/UCR unit 105 whereby theimage data is converted into a halftone area signal, a CMYK densitysignal that has its crosstalk component compensated for. Next, the imagedata in the form of the halftone area signal is γ-corrected by an outputγ-correction unit 106 to compensate for a linearity between the halftonearea signal and the output density as by dot gain processing. Then, theimage data is converted into binary data (referred to also as “bit mapdata”) for each color component by a binarization unit 107 and is outputfrom a host side interface unit 108 to a transmission line 201. The bitmap data transmitted from the host side interface unit 108 is taken intothe printer through a printer side interface unit 109. Further, in anH-V conversion unit 110 the bit map data has its output order convertedin accordance with a driving order of a print head. A mask generationunit 111 generates a thinning pattern (mask data) for multipass printingand gives it to a head driver 112, which in turn thins the converted bitmap data according to the mask data from the mask generation unit 111.Based on the thinned bit map data, the print head 113 is driven to ejectink during each of the scans with a paper feed operation performedbetween the scans. An image is thus formed by multipass printing.

As described above, in printing a single scan line the multipassprinting involves dividing the bit map data for the same scan line intoa plurality of parts and performing a plurality of passes and apredetermined amount of paper feed between the individual passes toeject ink from different nozzles of the print head in each of the passesaccording to the divided bit map data for the same scan line, thusforming an image for the same line. This multipass printing can reduceunevenness in printing due to dot landing deviations, variations in inkejection volume and ink penetration time differences.

The density conversion unit 103 and the output γ-correction unit 105 arenormally integrated into a lookup table (LUT), rather than beingprovided individually with processing circuits and software. Thisarrangement can shorten the processing time.

The conventional image processing method described above, however, hasthe following problem.

As the resolution of printed images and the printing speed in theprinter tend to increase, the amount of image data processed by theimage processing unit also increases. With the conventional method,however, the resolution conversion, density conversion, blackgeneration, masking/UCR, output γ-correction, binarization, H-Vconversion, and mask processing are all successively performedindependently. Hence, processing such a large amount of image dataaccording to the conventional method will take relatively long and thecircuit and the amount of calculations required will inevitably becomehuge.

Further, as the number of dots in a dot pattern that forms a pixelincreases, i.e., as the number of gray scale levels increases, thebinarization processing becomes more complicated and the amount ofcalculations increases.

Further, the memory capacity required for the H-V conversion increasesas the printer resolution and the width that can be printed in one passincrease or, in a more general term, as the number of nozzles in theprint head increases. This gives rise to another problem that when theresolution is enhanced, the memory capacity required for processingincreases in proportion to the square of the resolution.

Further, high-resolution printers tend to have an increased number ofnozzles integrated in the print head and make ink droplets ejected fromthese nozzles smaller to reduce dot diameters and thereby realize higherresolutions. Since the ink droplets ejected from the nozzles of such aprint head are very small, dot position variations due to landingdeviations are apt to become large relative to the dot diameter. Thuseven the multipass printing described above may not be able to eliminateimage quality degradations caused by printing variations. Further, themultipass printing often uses a checker pattern as the mask pattern inthe mask processing. When such a mask pattern is used, problems mayarise in which dots that need to be printed fail to be printed or dotprinting concentrates on a particular pass.

SUMMARY OF THE INVENTION

The present invention has been accomplished in light of the problemsdescribed above and it is an object of the present invention to providea printing system and a printing method with a low cost and a reducedprocessing time and with a capability to take full advantage of featuresof the multipass printing.

According to one aspect, the present invention provides a printingsystem, which uses a print head having a plurality of print elements,performs a plurality of scan operations of the print head over the samearea, and allocates different print elements to each of the plurality ofscans for printing, the printing system comprising: a code table fromwhich to output representative color codes based on input color data;and a pattern generation means for generating dot patterns of therepresentative color codes according to a correspondence between theinput color data and the representative color codes; wherein the dotpattern generation means stores the dot patterns and outputs in each ofthe plurality of scans the associated dot pattern.

According to another aspect, the present invention provides a printingmethod, which uses a print head having a plurality of print elements,performs a plurality of scan operations of the print head over the samearea, and allocates different print elements to each of the plurality ofscans for printing, the printing method comprising the steps of:outputting representative color codes from a code table based on inputcolor data; and generating dot patterns of the representative colorcodes according to a correspondence between the representative colorcodes and the dot patterns; wherein the step of generating the dotpatterns outputs in each of the plurality of scans the associated dotpattern.

With the printing apparatus and method described above, the processingtime can be shortened by replacing the image data with dot patterns ofthe representative color codes before printing.

The above and other objects, effects, features and advantages of thepresent invention will become more apparent from the followingdescription of embodiments thereof taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an image-processing unit according tothe present invention.

FIG. 2 is a diagram showing the structure of a code table.

FIG. 3 is a conceptual diagram showing a hue among cyan, magenta andyellow.

FIG. 4 is a diagram showing structure of an output density table.

FIG. 5 is a diagram showing the structure of a pattern table.

FIG. 6A is a diagram showing the structure of a dot pattern 1 and FIG.6B is a diagram showing the structure of a dot pattern 2.

FIG. 7A illustrates bit map data before being divided and FIG. 7Billustrates the structures of dot patterns divided into 2-pass printing.

FIG. 8 is a block diagram showing another example of theimage-processing unit.

FIG. 9 is a block diagram showing still another example of theimage-processing unit.

FIG. 10A illustrates bit map data before being divided and FIG. 10Billustrates the structure of dot patterns divided into 2-pass printing.

FIG. 11 is a block diagram showing a further example of theimage-processing unit.

FIG. 12A illustrates bit map data before being divided and FIG. 12Billustrates the structures of dot patterns divided into 2-pass printing.

FIG. 13 is a block diagram showing a conventional image-processing unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments applying the printing apparatus and printing methodaccording to the present invention will be described by referring to theaccompanying drawings.

Embodiment 1

FIG. 1 is a block diagram showing an image processing system as oneembodiment of this invention.

The image processing system comprises a system on the host side and asystem on the printer side. The host side system comprises an inputterminal 10, a density conversion unit 11, an addition unit 12, a spreadprocessing unit 13, a subtraction unit 14, a limiter 15, a quantizationunit 16, an output density table 17, a representative color codeconversion unit 18, and a host side interface unit 19. The printer sidesystem comprises a printer side interface unit 20, an H-V conversionunit 21, a pattern conversion unit 22, a selector 24, a pass controlunit 25, a rearrangement unit 26, a head driver 27, and a print head 28.

In this embodiment, for input data having a coordinate unit of 300 ppi(pixels per inch), dots that are actually formed are output at aresolution of 1200 dpi (dots per inch).

That is, one pixel is formed by using a 4×4-dot pattern. 4×4 dots aretreated as one unit and referred to also as a “block.”

The processing performed on the host side from the input terminal 10 tothe host side interface unit 19 is actually executed as printer driver'sprocessing.

A rough flow of processing in the image processing system of the aboveconstruction is as follows.

First, image data in the form of an RGB signal is entered into the inputterminal 10 for each pixel. The image data is converted into a CMYdensity signal by the density conversion unit 11. It is assumed herethat the CMY density signal does not exceed the reproduction range of arepresentative color described later. To the image data converted intothe density signal the addition unit 12 adds an error signal. The imagedata which is added the error signal then is limited by the limiter 15to prevent a possible gray scale inversion. Next, the limited image datais quantized by the quantization unit 16. In this embodiment, the imagedata for each pixel which consists of 8 bits in each CMY color isquantized into image data with four bits in each color by discarding thelower four bits of the data. The quantization is not limited to thismethod and other methods, such as rounding the lower four bits, may beused.

Next, the representative color code conversion unit 18 converts thequantized CMY 4-bit image data into a representative color code thatcorresponds to this data. In this embodiment, 256 different colorsuniformly distributed in a predetermined color space are selected as“representative colors” in advance. These representative colors areassigned “representative color codes” for identification. Then, colordata, a combination of each CMY four-bit data, is matched with arepresentative color code whose color is closest to the one presented bythe color data. This match or correspondence is set in a code table 181(see FIG. 2) in the representative color code conversion unit 18. Thatis, by referencing the code table 181 through an address based on theCMY 4-bit image data entered into the representative color codeconversion unit 18, the corresponding representative color code can bedetermined.

The representative color code thus determined is sent from the host sideto the printer side. That is, the representative color code is outputfrom the host side interface unit 19 through the transmission line 201of a printer cable to the printer side interface unit 20 and the H-Vconversion unit 21. In this embodiment, errors produced as a result ofcoding the image data are processed by the spread processing unit 13 andothers and the detail of the error processing will be explained later.The structure of the code table 181 and the representative color willalso be detailed later.

On the printer side, the H-V conversion unit 21 changes the order inwhich the representative color code data received is to be read out,according to the arrangement of the ink ejection nozzles in the printhead. That is, the H-V conversion unit 21 converts raster data sent fromthe host side for each line into data that corresponds to the nozzlearrangement of the print head, changing the horizontally arranged datainto vertically arranged data.

Next, the pattern conversion unit 22 outputs the dot pattern datacorresponding to the entered representative color code as the print headmakes each pass in the multipass scan operation. That is, the patternconversion unit 22 has pattern tables 22 a, 22 b (see FIG. 6) each ofwhich contains blocks of dot pattern data for each YMC color thatcorrespond to respective representative color codes. These patterntables 22 a, 22 b contain dot patterns for each of two passes in themultipass printing of this embodiment as described later. The structureof the pattern tables and the multipass printing based on the patterntables will be detailed with reference to FIG. 6 and FIG. 7.

Next, of the dot pattern data output from the two pattern tables 1 and2, the selector 24 selects and outputs the dot patterns from the patterntable specified by a control signal from the pass control unit 25. Inthis way, in the multipass printing consisting of two passes, it ispossible to output the dot patterns that correspond to each pass. Next,the rearrangement unit 26 rearranges the dot patterns in the order inwhich they are to be output from the print head 28. The head driver 27according to the rearranged dot patterns drives the print head 28 forprinting. The conversion into the output dot patterns by the patternconversion unit 22 and the rearrangement into the head drive data by therearrangement unit 26 are performed in synchronism with a head drivesignal. The CMYK heads are spaced a predetermined distance from eachother in many printing apparatus. Thus, in these apparatus theprocessing performed in and following the H-V conversion unit 21 isexecuted for each color independently. That is, the tables in thepattern conversion unit 22 are provided for each color, and for eachrepresentative color code entered, dot patterns, one for each C, M, Yand K, are output successively in synchronism with the head drivesignal.

During the processing described above, the errors produced when therepresentative color code conversion unit 18 changes the image data fromthe CMY density signal to the representative color code is processed asfollows. The output density table 17 stores colors reproduced by theprinter of this embodiment, i.e., density data for Y, M, C which areobtained by printing dot patterns corresponding to the representativecolor code, as will be described with reference to FIG. 4. The outputdensity table 17 outputs the reproduced color data corresponding to therepresentative color code entered from the representative color codeconversion unit 18. The density data of each of Y, M, C can be obtainedby such techniques as optical density detection and color measurement.The subtraction unit 14 determines a difference between the reproducedcolor and the input color, i.e., calculates an error of the outputdensity with respect to the input density for each of Y, M and C. Thespread processing unit 13 generates error signals for surrounding pixelsto spread the error over the surrounding pixels by a known errorspreading method. The error signals thus generated are added to theimage data by the addition unit 12 as described earlier.

According to the flow described above, the input image data is processedto perform the 2-pass printing using the print head 28.

As described above, the image processing using the representative colorcode according to this embodiment enables the color processing, rangingfrom the conventional density conversion to the output γ-correction asdescribed with reference to FIG. 13, and the successive binarizationprocessing to be performed integrally. The representative color code andthe representative color will be described in more detail in thefollowing.

The representative colors, as explained earlier, are selected in advancefrom among colors uniformly distributed in a predetermined color space.

In the image processing system of this embodiment, as already explained,the data processing for each pixel to print C, M, Y, K dot patterns of1200 dpi is performed using 300 ppi coordinates. Thus, the dot patternhas 4×4 dots for each pixel. To represent C, M, Y, K and a colorproduced from a combination of C, M, Y, K essentially requires 4×4×4=64bits. In this embodiment, however, an 8-bit representative color code isused. Thus, data compression by a factor of 8/64=⅛ is possible. Comparedwith the 2⁶⁴ different colors represented by the 64 bits, the number ofcolors that can be perceived by humans is significantly small. Withprecision variations in output dot position, dot diameter and dotdensity also taken into consideration, it is possible to significantlyreduce the number of colors and still produce an output image that ishardly different from an image based on 2⁶⁴ colors if the representativecolors are properly selected. In this embodiment the following method isused in selecting representative colors and replacing the input imagedata with the selected representative colors to reduce the number ofcolors for use in image formation and compress the image data. Becausethe image data can be compressed in this way, the volume of datatransmitted over the transmission line 201 can be reduced. This can notonly shorten the transmission time but greatly reduce the memorycapacity required by the H-V conversion unit 21.

The representative colors may be selected in the following two methods.A first method involves printing patches of all possible dot patterns onthe printer with a 4×4-dot block shown in FIG. 5 taken as a unit,measuring the colors of the printed patches of each pattern, andselecting, as representative colors from among all the measured dotpatterns, those colors that are uniformly distributed in a predeterminedcolor space (for example, an input CMY space and an L*a*b* space).

Since there are 2⁶⁴ dot patterns in all as described above, measuringthe colors of all dot patterns, as specified in the first method, is notpractical.

A second method involves modeling a relation between the dot patternsand the colors formed by printing the dot patterns, calculating thecolors corresponding to all dot patterns, and selecting asrepresentative colors those colors that are uniformly distributed in apredetermined color space. In more concrete terms, the number ofpatterns is reduced by placing a limitation on the number of mixedcolors and by eliminating those patterns that are found identicalthrough rotation and mirroring of images. Then the known modeling using,for example, a color halftone model is performed to further reduce thenumber of patterns down to a level where the color measurements of theavailable patterns are feasible. This is followed by the patches beingprinted and actually measured in the same way as the first method. Thenthose colors uniformly distributed in a predetermined space color areselected as representative colors. When printed colors are measured,some limitations (rules) may be imposed which include eliminating thosedot patterns with large measurement variations as unstable patterns andpreventing those patterns of sparsely distributed dots from beingremoved in order to alleviate the granularity of highlight areas.

The reduction in the number of representative colors by using the colorhalftone model is achieved as follows.

A test pattern is printed for each combination of overlapping ink colorsand measured in advance to determine a density (hereinafter referred toas an “assessment value”) for each combination of overlapping inks.Further, by taking statistics of dots the printer forms on a printmedium, an equivalent dot diameter is determined for each ink.

Then, an area ratio of each combination of overlapping inks to each dotpattern is determined from the equivalent dot diameter of the ink, thedot pitch determined from the resolution, and the dot arrangement in thedot pattern. A product of the area ratio and the assessment value foreach combination of overlapping inks is calculated and a sum of theseproducts in each dot pattern is taken to be a reproduced color value ofthe dot pattern. Next, a difference is taken between the reproducedcolor values of different dot patterns. If a combination of dot patternsis found for which this difference is less than a predetermined level(ΔE), one of the dot patterns whose density is lower, i.e., whosereproduced color value is smaller, is removed from the candidates forthe representative colors. This process is repeated. If a predeterminednumber or more of the representative color candidates still remain evenafter the combinations whose reproduced color value difference is lessthan the predetermined level (ΔE) have been processed and eliminated,the value of the predetermined level (ΔE) is increased and theabove-described process is repeated until the number of representativecolor candidates falls below the predetermined number. Therepresentative colors are determined in this manner.

As described above, in this embodiment, the representative colors arematched to their 4×4-dot patterns. FIG. 5 shows the correspondencebetween the representative color codes generated by the representativecolor code conversion unit 18 (see FIG. 1) and the dot patterns for eachof C, M, Y, K colors. As shown in the figure, the selected 256representative color codes are matched to dot patterns for each CMYKcolor. Printing these patterns produce the corresponding representativecolors.

The representative color codes are output from the representative colorcode conversion unit 18 for each input color data C, M, Y. That is, thecode table 181 of the representative color code conversion unit 18assigns the color represented by four bits of each CMY with arepresentative color code that should be output. Each of the CMY colorshas 16 possible representations, and thus a total of 16×16×16 differentcolor representations is possible. FIG. 2 and FIG. 3 illustrate aconcept of the code table 181. The position of each lattice point in acubic lattice is represented by the four bits for each of C, M, Y. Therepresentative color codes are mapped in these lattice point.

The mapping of the representative color code in the code table isperformed as follows. First, the 4-bit density data for C, M, Yrepresenting a lattice point are converted into quantized representativevalues. The quantized representative value is produced by a conversionformula of “sum of weight of each bit+quantization step/2” whichconverts a 4-bit value into an 8-bit value. The lattice point which isconverted into quantized representative values is expressed as x(C′, M′,Y′). This method is used because, in processing the input data, theconversion is done by the quantization unit 16 by discarding the lowerfour bits of the input image data. If the quantization unit 16 performsrounding rather than omitting the lower four bits, a value with thelower four bits set to 0 may be used.

Next, the color measurement value of the representative color that isrelated by the output density table 17 described later is given as a(C,M, Y). The distance between the lattice point x and the colormeasurement value a is determined as follows:r ²=(a−x)²=(C−C′)²+(M−M′)²+(Y−Y′)²  (1)

A representative color is determined which corresponds to a value a forwhich the distance r determined above is minimum. This representativecolor is taken as the representative color code that is to be mapped inthe lattice point.

An error represented as the distance r is spread over the adjoiningpixels by the spread processing unit 13 using the conventionally knownerror spreading method. Although in this embodiment the square of thedistance between the color measurement value of the representative colorand the quantized representative color is determined, it is alsopossible to determine an absolute value of the distance and thendetermine a representative value for which this absolute value isminimum. While the quantized representative value uses a center value ofthe quantized value, it is not limited to the center value. Thequantized representative value may be a wholly or partly shifted value.

Further, although in the above embodiment an input to the output densitytable 17 is used as the representative color code, the CMY signal afterquantization may be used as the code. In that case, the code table 181and the output density table 17 may be integrated as one table so thatthe representative color code and the output density value (colormeasurement value) can be obtained in one LUT search. This furtherenhances the speed of the processing. If the representative color codeis 8 bits long, because the output density value is 24 bits (8×3=24)long, the code table 181 will have a 32-bit (8+24=32) output, whichprovides good matching when the host performs processing in 32 bits.Furthermore, for an input outside the color reproduction range, thevalue is corrected before being stored in order to prevent colorreproduction errors from accumulating. This can prevent possible colordeviations outside the color reproduction range of the printer.

Next, we will explain about the output density table (see FIG. 1) usedto determine errors that are produced as a result of converting an inputcolor into a representative color code.

FIG. 4 conceptually illustrates the output density table 17 of thisembodiment.

The output density table 17 matches a representative color code to acombination of 8-bit CMY densities. The density data to be stored inthis table can be determined as follows. First, patches of a dot patterncorresponding to each representative color as shown in FIG. 5 are outputon the printer of this embodiment and the color measurement is made ofthese patches. In this color measurement patch positions are separatedfrom each other, and two or more measurements are taken and averaged.Then, the average measurement values (X, Y, Z) are substituted into thefollowing equations to convert them into an RGB signal in an NTSC space:R=(1.910X−0.532Y−0.288Z)/100G=(−0.985X+1.999Y−0.028Z)/100B=(0.058X−0.118Y−0.898Z)/100  (2)

Next, the RGB signal is substituted into the following equations todetermine an RGB density (Dr, Dg, Db):Dr=−log₁₀ (R)Dg=−log₁₀ (G)Db=−log₁₀ (B)  (3)

Then, the density value is normalized for conversion into an input colorspace:C=(Dr−Dmin)×255/(Dmax−Dmin)M=(Dg−Dmin)×255/(Dmax−Dmin)Y=(Db−Dmin)×255/(Dmax−Dmin)  (4)

The measured value obtained through conversion as described above isstored at an address of the corresponding representative color code inthe output density table 17. While the method of conversion into theinput color space uses a log function for conversion into the RGBdensity, it is not limited to this example but may use an LUT. When onewishes to shift the output color of the printer from the input color, asin a gamut compression, this can be achieved by counter-correcting thevalue to be stored in the output density table 17. For example, when agamut compression is to be made, the value of the reproduction colordata is shifted slightly at a time toward the outside of the colorreproduction range of the printer so that the reproduction color dataencloses the color reproduction range in the input color space. Thereproduced CMY colors for blank(white) are all set to 0.

Next, an explanation will be given to the pattern conversion unit 22(see FIG. 1), a feature of this invention, which stores dot patternscorresponding to respective representative color codes.

The pattern tables 1 and 2 in the pattern conversion unit 22 comprise,as described earlier, representative color codes and their correspondingdot patterns. Printing is done according to the dot patterns asdescribed above.

In the multipass printing, the print data is conventionally thinned by apredetermined mask pattern for each pass. Because the thinning is donein units of dots, in areas where the density is low there is apossibility that the thinning (mask) pattern and the print pattern mayinterfere with each other, resulting in the number of printed dotsbecoming uneven between the passes. To deal with this problem, thisembodiment prepares the same number of pattern tables as that of thepasses and stores these pattern tables by relating them with therespective passes. In printing, the dot patterns are output according tothe representative color codes stored in the pattern table associatedwith the current pass being executed, thus printing an image based onthis dot pattern data.

Next, how the pattern table is divided will be explained in more detailby taking a two-pass printing for example.

FIG. 5 shows the correspondence between the representative color codesand the dot patterns before the pattern table is divided. Dot patternsfor four CMYK colors are each matched with a representative color code.In the 2-pass printing, the dot patterns shown in the figure are eachdivided into two dot patterns of FIG. 6A and FIG. 6B. The dot divisionis performed by ensuring that the necessary dots are divided into bothpattern tables. In the case of a representative color 255, for example,simply dividing the cyan and magenta patterns into two tables accordingto the checker pattern will result in the dots being heavily allocatedonly to one table. This embodiment, therefore, takes steps to ensurethat the dots are equally divided between the two pattern tables 22 a(see FIG. 6A) and 22 b (FIG. 6B).

The dot outputs from the two tables in principle do not overlap. In therepresentative color 255, however, the dot output is set so that severaldots overlap each other. That is, the total number of dots in the twopattern tables 22 a, 22 b associated with the representative color code255 is 20, which is larger than the total number of dots of 16 beforethe pattern table is divided. This means that several dots in the blockoverlap between the first pass and the second pass. This is done toincrease the density of solid black.

By dividing the dot pattern table according to the number of passes inthe multipass printing, it is possible to change the dividing method,the number of dots to be ejected, and the color overlapping for eachrepresentative color.

The image processing is performed in this way using the code table 181,output density table 17 and pattern tables 22 a, 22 b prepared inadvance to perform the multipass printing. Next, the process of printingwill be explained. In this embodiment the multipass printing is adopted.

FIG. 7 shows a concept for the multipass printing according to thisembodiment.

FIG. 7A shows bit map data before being divided. In this embodiment thebit map data is printed in two passes, so the print data is dividedaccording to the representative color codes into two print data, asshown at top left and bottom left in FIG. 7B, by the pattern conversionunit 22. In this embodiment, the dot pattern division is made in such away that the output is produced every other dot and not produced at thelast column with respect to the direction of main scan. Here, the dotdiameter is set to about two times the dot pitch, and when the number ofoutput dots in the 4×4-dot block is 8 or less, the pattern table 22B(dot pattern 2 in FIG. 6) is set to blank. This step is taken toeliminate possible degradations of print quality that may be caused bylanding deviations and paper feed errors. Numbers in the figure identifythe pattern table to be used. The selector 24 uses an output from thepattern table 22 a (see FIG. 6A) when the number is “1” and an outputfrom the pattern table 22 b (see FIG. 6B) when the number is “2.” Eachsquare in the figure represents one block. The final print output is animage produced by combining a top right pattern and a bottom rightpattern of FIG. 7B. Although the final print output image differs fromthe original bit map data, the adverse effects from the landingdeviations are alleviated because the dot diameter is set to about twotimes the dot pitch and the blocks in an intermediate-to-low-densityrange where density variations appearing as lines become conspicuous areoutput in a single pass.

Further, in the dot pattern showed in FIGS. 6A and 6B, the mask block isformed 4×4-dot, such that has a resolution of 300 dpi. Therefore, dotsin one block within 300 dpi are formed in one pass. Hence, the printingunevenness that occurs within the one block of 300 dpi is mostlyattributable to the head characteristic alone and thus corrected by the300-dpi mask block. The printing variations due to an accuracy ofprinting mechanics, such as paper feeding mechanics, occur at theboundary of the block of 300 dpi and are distributed over an entireimage. Therefore in this embodiment, to reduce density variationsappearing as horizontal lines in the output image such as the printingunevenness that is occurred by paper feeding, the dot density of the dotpattern in the vertical direction is increased, such as code “254” or“255” in FIGS. 6A and 6B.

The output density table 17 uses values actually measured during themultipass printing, so that the density deviations caused by differingdot patterns are corrected each time the output density table 17 isupdated.

In this embodiment, because the CMY density data is directly convertedinto representative color codes by using the code table 181 and sent tothe printer side, the conventionally performed processing, such asresolution conversion, masking/UCR conversion and output .gamma.correction, can be integrated into a simple arrangement. The code table181 is generated based on the output density table 17 that is obtainedby measuring the colors of patches, so the black generation processingdoes not need to be executed independently, thus reducing the processingtime compared with the conventional arrangement.

In this embodiment, because the dots are not contiguous in the main scandirection, when the head is driven at the same ejection frequency, thescan speed in the main scan direction can be doubled.

Embodiment 2

FIG. 8 shows a block diagram of a second embodiment of the imageprocessing system according to the present invention. The configurationon the host side is the same as that of the embodiment 1, whereas on theprinter side the pattern tables used in the pattern conversion unit 23following the H-V conversion unit 21 differ from those of theembodiment 1. After the selector 29 there is provided a blankinterpolation unit 30 before the rearrangement unit 26.

In this embodiment, the pattern tables 23 a, 23 b omit those dots thatare always blank in the pattern tables 22 a, 22 b of the embodiment 1.By removing blank dots the capacity of the pattern tables can bereduced.

The pattern conversion unit 23 converts the input representative colorcode into an output dot pattern associated with the pass by using thepattern tables 23 a, 23 b.

The selector 29, according to a control signal from the pass controlunit 25, selects one dot pattern from the outputs of the pattern tables23 a, 23 b.

The blank interpolation unit 30, according to the control signal fromthe pass control unit 25, adds blanks omitted from the pattern tables 23a, 23 b to the dot pattern entered from the selector 29, thus generatinga dot pattern of normal size. The processing following the interpolationof blanks is similar to that of embodiment 1 and thus its explanation isomitted.

How the blanks are omitted from the pattern tables 23 a, 23 b will bedescribed in detail.

In the high-speed 2-pass printing shown in FIG. 7, for example, thepattern table 22 a leaves even numbered columns always blank and thepattern table 22 b leaves the right end column always blank. Thus, thepattern table 23 a stores data of the pattern table 22 a removed of theeven-numbered columns, and the pattern table 23 b stores data of thepattern table 22 b removed of the right end column. Then, when theselector 29 selects the pattern table 23 a, the blank interpolation unit30 adds blanks to the even-numbered columns and, when the selector 29selects the pattern table 23 b, adds blanks to the right end column toform dot patterns as shown at top right and bottom right in FIG. 7. Byremoving blanks in this way the capacity of the pattern table 23 a isreduced to one-half that of the pattern table 22 a and the capacity ofthe pattern table 23 b to three fourths that of the pattern table 22 b.

Embodiment 3

This embodiment performs a mask-based thinning for each printing passwithout using pattern tables.

FIG. 9 shows a block diagram of the image processing system according tothis embodiment. The host side has the same configuration as that ofembodiment 1, whereas on the printer side a mask processing unit 31 anda mask generation unit 32 are provided following the H-V conversion unit21 and a pattern processing unit 33 is provided after the maskprocessing unit 31.

The mask generation unit 32 generates a thinning pattern (also referredto as mask data) associated with the current output pass in units ofblocks at 300 dpi and supplies the thinning pattern to the maskprocessing unit 31. The mask processing unit 31 masks the representativecolor code output from the H-V conversion unit 21 with the mask patternof the mask generation unit 32. In this embodiment, because therepresentative color code for the blank is set to 0, the representativecolor codes for the blocks that need to be masked are replaced with 0.The pattern processing unit 33 stores dot patterns for therepresentative colors and converts the representative color code outputfrom the mask processing unit 31 into the associated dot pattern. Then,this dot pattern is printed.

In the multipass printing, the number of mask patterns generated by themask generation unit 32 corresponds to the number of passes.

FIG. 10 shows a concept of the multipass printing according to thisembodiment.

In this embodiment, masking is done in units of 4×4-dot blocks, asdescribed earlier.

FIG. 10A shows bit map data for a final image to be formed.

In this embodiment, since this bit map data is printed in two passes,the masking is done by using two mask patterns shown at top left andbottom left in FIG. 10B. A final image formed is a combination of a topright pattern and a bottom right pattern in FIG. 10B.

In this embodiment, the mask has a resolution of 300 dpi and dots within300 dpi are formed in one and the same pass. Hence, the printingunevenness that occurs within the 300 dpi is mostly attributable to thehead characteristic alone. The printing variations due to mechanicalaccuracy occur concentratedly at the boundary of 300 dpi and aredistributed over an entire image, so the printing variation sensitivitywith respect to the mechanical accuracy increases as the dot diameterapproaches 300 dpi. Since the representative color code for the blank isset to 0, the mask processing unit 31 can be built by taking a logicalproduct (AND) of the representative color code and the mask pattern. Ifthe representative color codes for the blanks are all set to “1,” themask processing unit 31 can be formed by taking a logical sum (OR).

Further, because the masking is done in units of blocks, interferences(beats) between the mask pattern and the bit map, often experienced withthe conventional system, do not easily occur. For example, when thepattern shown at the left in FIG. 10 is masked with a checker pattern orreversed check pattern in units of dots, there is a possibility that apattern may be produced which causes the dots to appear concentratedlyon only a particular pass or which eliminates the dots that need to beprinted. However, the masking in units of blocks can disperse the dotsuniformly as shown at the top right and bottom right.

Embodiment 4

FIG. 11 shows a block diagram of the image processing system accordingto embodiment 4. The host side has the same configuration as inembodiment 1, whereas the printer side has after the H-V conversion unit21 a code conversion unit 34 and a pattern table unit 35.

The code conversion unit 34 converts, according to a control signal fromthe pass control unit 25, the representative color code output from theH-V conversion unit 21 into an address in the pattern table 35 for eachpass. The pattern table 35 stores all dot patterns after they aredivided between passes. Upon receiving an address signal from the codeconversion unit 34, the pattern table unit 35 generates a 4×4-dotpattern and outputs it to the rearrangement unit 26.

The code conversion unit 34 has a logic or LUT. If the input code is 8bits long and the number of passes is 2, then the code conversion unit34 can be constructed of 512×8 LUTs. The pattern table unit 35 needs tostore only those patterns that are to be activated. This reduces thecapacity of this unit.

FIG. 12 shows a concept of the multipass printing according to thisembodiment.

FIG. 12A shows bit map data for a final image to be formed.

In this embodiment because this bit map data is printed in two passes,it is divided into two print data as shown at top left and bottom leftin FIG. 12B.

This is achieved by changing the addresses in the pattern table 35 inresponse to the control signal from the pass control unit 25. A finalprint output is an image produced by combining a top right pattern and abottom right pattern in FIG. 12B.

In this embodiment, blocks with a mask “2” in other than solid colorareas are blank. Thus, the code conversion unit 34 outputs blankaddresses (in this embodiment “0”) for all blocks with the mask “2” inother than the solid color areas and, for all blocks with the mask “1”,outputs addresses corresponding to the dot patterns of therepresentative colors. For blocks with mask “2” in the solid colorareas, addresses corresponding to the dot patterns of intermediatedensities are output. Because this arrangement increases the number ofsolid color blocks, it is effective in enhancing the contrast of blackcharacters.

Such an arrangement is not limited to the example of FIG. 12 and avariety of multipass printing including the patters shown in FIG. 7 andFIG. 10 can be realized by changing a combination of the code conversionunit 34 and the pattern table unit 35.

Although in the embodiments of the present invention the 2-pass printinghas been described, the invention can also be applied to the multipassprinting with two or more passes. Further, while the mask pattern andthe pass control pattern have been described to be a checker pattern,any other pattern may be used.

The present invention achieves distinct effect when applied to arecording head or a recording apparatus which has means for generatingthermal energy such as electrothermal transducers or laser light, andwhich causes changes in ink by the thermal energy so as to eject ink.This is because such a system can achieve a high density and highresolution recording.

A typical structure and operational principle thereof are disclosed inU.S. Pat. Nos. 4,723,129 and 4,740,796, and it is preferable to use thisbasic principle to implement such a system. Although this system can beapplied either to on-demand type or continuous type ink jet recordingsystems, it is particularly suitable for the on-demand type apparatus.This is because the on-demand type apparatus has electrothermaltransducers, each disposed on a sheet or liquid passage that retainsliquid (ink), and operates as follows: first, one or more drive signalsare applied to the electrothermal transducers to cause thermal energycorresponding to recording information; second, the thermal energyinduces sudden temperature rise that exceeds the nucleate boiling so asto cause the film boiling on heating portions of the recording head; andthird, bubbles are grown in the liquid (ink) corresponding to the drivesignals. By using the growth and collapse of the bubbles, the ink isexpelled from at least one of the ink ejection orifices of the head toform one or more ink drops. The drive signal in the form of a pulse ispreferable because the growth and collapse of the bubbles can beachieved instantaneously and suitably by this form of drive signal. As adrive signal in the form of a pulse, those described in U.S. Pat. Nos.4,463,359 and 4,345,262 are preferable. In addition, it is preferablethat the rate of temperature rise of the heating portions described inU.S. Pat. No. 4,313,124 be adopted to achieve better recording.

U.S. Pat. Nos. 4,558,333 and 4,459,600 disclose the following structureof a recording head, which is incorporated to the present invention:this structure includes heating portions disposed on bent portions inaddition to a combination of the ejection orifices, liquid passages andthe electrothermal transducers disclosed in the above patents. Moreover,the present invention can be applied to structures disclosed in JapanesePatent Application Laid-Open Nos. 59-123670 (1984) and 59-138461 (1984)in order to achieve similar effects. The former discloses a structure inwhich a slit common to all the electrothermal transducers is used asejection orifices of the electrothermal transducers, and the latterdiscloses a structure in which openings for absorbing pressure wavescaused by thermal energy are formed corresponding to the ejectionorifices. Thus, irrespective of the type of the recording head, thepresent invention can achieve recording positively and effectively.

The present invention can also be applied to a so-called full-line typerecording head whose length equals the maximum length across a recordingmedium. Such a recording head may consist of a plurality of recordingheads combined together, or one integrally arranged recording head.

In addition, the present invention can be applied to various serial-typerecording heads: a recording head fixed to the main assembly of arecording apparatus; a conveniently replaceable chip type recording headwhich, when loaded on the main assembly of a recording apparatus, iselectrically connected to the main assembly, and is supplied with inktherefrom; and a cartridge type recording head integrally including anink reservoir.

It is further preferable to add a recovery system, or a preliminaryauxiliary system for a recording head as a constituent of the recordingapparatus because they serve to make the effect of the present inventionmore reliable. Examples of such recovery system are a capping means anda cleaning means for the recording head, and a pressure or suction meansfor the recording head. Examples of the preliminary auxiliary system area preliminary heating means utilizing electrothermal transducers or acombination of other heater elements and the electrothermal transducers,and a means for carrying out preliminary ejection of ink independentlyof the ejection for recording. These systems are effective for reliablerecording.

The number and type of recording heads to be mounted on a recordingapparatus can be also changed. For example, only one recording headcorresponding to a single color ink, or a plurality of recording headscorresponding to a plurality of inks different in color or concentrationcan be used. In other words, the present invention can be effectivelyapplied to an apparatus having at least one of the monochromatic,multi-color and full-color modes. Here, the monochromatic mode performsrecording by using only one major color such as black. The multi-colormode carries out recording by using different color inks, and thefull-color mode performs recording by color mixing.

Furthermore, although the above-described embodiments use liquid ink,inks that are liquid when the recording signal is applied can be used:for example, inks can be employed that solidify at a temperature lowerthan the room temperature and are softened or liquefied in the roomtemperature. This is because in the ink jet system, the ink is generallytemperature adjusted in a range of 30° C.-70° C. so that the viscosityof the ink is maintained at such a value that the ink can be ejectedreliably.

In addition, the present invention can be applied to such apparatuswhere the ink is liquefied just before the ejection by the thermalenergy as follows so that the ink is expelled from the orifices in theliquid state, and then begins to solidify on hitting the recordingmedium, thereby preventing the ink evaporation: the ink is transformedfrom solid to liquid state by positively utilizing the thermal energywhich would otherwise cause the temperature rise; or the ink, which isdry when left in air, is liquefied in response to the thermal energy ofthe recording signal. In such cases, the ink may be retained in recessesor through holes formed in a porous sheet as liquid or solid substancesso that the ink faces the electrothermal transducers as described inJapanese Patent Application Laid-Open Nos. 54-56847 (1979) or 60-71260(1985). The present invention is most effective when it uses the filmboiling phenomenon to expel the ink.

Furthermore, the ink jet recording apparatus of the present inventioncan be employed not only as an image output terminal of an informationprocessing device such as a computer, but also as an output device of acopying machine including a reader, and as an output device of afacsimile apparatus having a transmission and receiving function.

As described above, in the printing apparatus and printing methodaccording to the present invention, the colors used in forming an imageare limited to a predetermined number of color patterns calledrepresentative colors in advance. On the host side, the image datareceived is divided in units of blocks, an appropriate representativecolor is determined for each block, and the representative colors andthe associated representative color codes are sent to the printer side.On the printer side, the representative color codes are converted intodot patterns for each pass by performing a series ofprocessing—resolution conversion, masking/UCR processing, outputγ-correction and binarization processing. These processing can beintegrated into a simple circuit arrangement, allowing for a higherspeed of processing.

In an N-pass printing, the output dot pattern is blanked every N dots inthe main scan direction and N dots from the last column are alsoblanked. This arrangement can increase the main scanning speed by Ntimes.

Further, by making a setting such that the total number of dots in thedivided dot patterns that are to be output in the multipass printing isgreater than the number of output dots in the dot pattern before beingdivided, the density of solid color areas can be increased, improvingthe contrast.

The dot resolution of the output dot pattern in the paper feed directionis set higher than that in the main scan direction to reduce theconspicuousness of density variations appearing as horizontal linescaused by paper feed variations.

Further, in storing the dot patterns of the representative colors, thosedots that are always blank are omitted. This can reduce the capacityrequired for the pattern table.

Further, because the thinning is done in units of blocks during themultipass printing, the dot patterns of the representative colors areoutput as is for those blocks that are to be printed. This can simplifythe processing and also reduce the capacity required for the patterntable. Furthermore, because the code data received is processed (byadding blanks) according to a control signal (pass signal) beforeconverting it into dot patterns, the capacity of the pattern table canalso be reduced.

The present invention has been described in detail with respect topreferred embodiments, and it will now be apparent from the foregoing tothose skilled in the art that changes and modifications may be madewithout departing from the invention in its broader aspect, and it isthe intention, therefore, in the apparent claims to cover all suchchanges and modifications as fall within the true spirit of theinvention.

1. A printing system, which uses a print head having a plurality ofprint elements and performing a scan operation over the same area aplurality of times, the printing system comprising: a code table fromwhich to output color codes based on values of respective input colorsfor a pixel; a pattern generation means having a plurality of convertingtables respectively for the plurality of times of scan operation, fromwhich to output dot patterns each of the plurality of times of scanoperation, wherein the individual converting tables are built accordingto the color codes outputted from said code table and the dot patternsformed by combing all the converting tables correspond respectively tothe color codes.
 2. A printing system according to claim 1, furthercomprising a diffusion unit for determining values of reproductioncolors that are obtained by measuring dot patterns printed correspondingto the respective color codes, based on color codes outputted from saidcode table, and diffusing differences between the values of reproductioncolors and the values of input colors of the pixel into other pixels. 3.A printing system according to claim 1, wherein each of the plurality ofconverting tables stores dot patterns for the respective input colors ina block unit.
 4. A printing system according to claim 1, furthercomprising: a selector that selects the converting table correspondingto the scan operation from the plurality of converting tables, based ona control signal from a pass control unit.
 5. A printing method ofperforming printing, using a print head having a plurality of printelements and performing a scan operation over the same area a pluralityof times, said method comprising: an output step of outputting colorcodes from a code table based on values of respective input colors for apixel; a preparing step of preparing a plurality of converting tablesrespectively corresponding to the plurality of times of scan operationaccording to the color codes outputted from said code table; and a dotpattern generation step of generating dot patterns in each of theconverting tables, wherein the dot patterns formed by combining all theconverting tables correspond respectively to the color codes.
 6. Aprinting system, which uses a print head having a plurality of printelements and performing a scan operation over an area a plurality oftimes, the printing system comprising: an output density table thatoutputs multiple-value data showing densities of respective input colorscorresponding to a color code based on image data of the respectiveinput colors for a pixel; and pattern generation means having aplurality of converting tables respectively corresponding to a number oftimes of scan operation, each of the plurality of converting tablesoutputting dot patterns based on the multiple-value data outputted fromsaid output density table.